The molecular mechanisms of chromosome replication and genetic recombination have not yet been elucidated. Although a variety of enzymes involved in DNA metabolism have been purified and characterized, these biological processes have not been duplicated in a cell-free system. This difficulty in part reflects a lack of information concerning the reaction mechanism of the purified enzymes as well as the necessity of identifying other enzymes and structural components. The detailed mechanism of action of enzymes as the DNA polymerases, DNA ligases, and nucleases will be studied in order to design experiments to determine their role in vivo. Particular emphasis will be placed on the use of an in vitro system for studying phage T7 DNA replication. Complementation assays will be used to identify and to purify the products of the T7 genes 2, 4, and 5 all of which are required for T7 DNA replication. In addition host proteins, such as the DNA polymerases and tsnC protein, will be purified and studied in order to determine their specific roles in T7 DNA replication.